This invention relates to a local area network (LAN) telephone system modified to provide emergency telephone service in the case of power failure or LAN processor failure.
In a typical analog telephone system used in homes and small businesses, extension phones are connected in parallel across the telephone line conductors. In such a typical system, as shown in FIG. 1, a telephone line 110 comprising a wire pair extends from a public switched telephone network (PSTN) to several conventional analog telephones 121, 122 and 123 within the premises 102 of a home or business. The telephones 121-123 are connected in parallel across the wire pair of the telephone line 110. Power for the analog telephones is supplied by the telephone company on the telephone line from an uninterruptable power source (UPS) and the analog telephones require no other power source. Each of the telephones 121-123 can be used to make a telephone call or receive a telephone call. The system is very reliable and can and does provide uninterrupted telephone service in all but the worst national disasters. The telephone system will remain fully operative in most instances when electric power to the business or home is interrupted. However, the analog telephones 121-123 cannot be used to make calls to each other, cannot be used as an intercom system, and cannot transfer calls or hold calls or perform other functions provided by most modern telephone systems.
To increase the functionality of their telephone system, many small businesses and some private homes employ a private branch exchange (PBX) in a system, such as that shown in FIG. 2. In the system of FIG. 2, one or more telephone lines 101 are terminated in a PBX server 210. The PBX server 210 is separately connected by connections 221, 222 and 223 to telephones 231, 232 and 233, respectively. The connections 221, 222 and 223 can be analog or digital, but the connections are point to point, each allowing communication only between the PBX and the corresponding telephone. The configuration of FIG. 2 allows for rich functionality because the PBX server 210 can route incoming calls to any one of the telephones 231-233 as well as let the telephones 231-233 make calls to each other, provide a hold function, provide a forwarding function, provide voice mail and many other modem telephone functions. This configuration, however, has a disadvantage in that the PBX server 210 must be powered from an external power source and if power fails or if the PBX server 210 fails, the phone service for all of the phones is interrupted.
Another typical PBX system is shown in FIG. 3. In this system, one or more telephone lines 101 is terminated in a PBX server connected to telephones 321, 322 and 323 over a local area network (LAN) 320, which may be a coax cable which is connected in parallel to the telephones 321, 322 and 323. Alternatively, instead of the coax cable, the LAN network may be twisted pairs, such as an Ethernet 10-base T system. In any case, the telephones 321, 322 and 323 are logically connected in parallel and can receive all data and transmit to all other end points on the LAN including transmissions via logically transparent repeaters, hubs, switches and routers. Like the PBX arrangement of FIG. 2, the LAN system in FIG. 3 allows a high degree of functionality and, in addition, provides the capability of having computers connected as end points and permits computers to be used as telephones, telecopiers and answering devices. However, like the PBX server in FIG. 2, the PBX server 310 of the system in FIG. 3 must also receive power from an external source and it is therefore suffers from the same weakness in that service to all of the telephones and other end points can be cut off in the event of power failure or failure of the PBX server.
The above described disadvantages of the systems of FIGS. 2 and 3 are not major problems in large businesses because there, power can be made redundant with UPS devices and, typically, a professional maintenance staff is available. In such large businesses, PBX functions are seen as being indispensable and the businesses willingly trade the lower reliability of the PBX systems for the enhanced telephone systems that they receive from the PBX systems. However, typically, in small businesses and in private homes, there is no maintenance staff and an UPS is an unwanted expense. Accordingly, there is a need for small businesses and private homes for a telephone system which has the features of a PBX system but yet has the reliability of the old fashioned analog telephone system shown in FIG. 1.
The system of the present invention is implemented in a LAN telephone system in which a PBX is used to route telephone calls to or from telephones or other telephonic devices of the system. In accordance with the invention, fallback adapters are provided to automatically respond to a power failure and also automatically respond to a failure of the PBX. When a power failure occurs or a PBX failure occurs, the fallback adapters directly connect the telephones to a telephone line or telephone lines of the system to enable the telephones to be used as analog phones and make and receive calls directly without using the PBX. Power failure is detected in the fallback adapters by a relay which defaults to a position to connect the telephones directly to the telephone line. In addition, the fallback detectors are provided with controller logic which detect the presence of a ringing signal on the telephone line and the failure of the PBX to respond to the ringing signal. In addition, the fallback detectors detect the failure of the PBX to respond when a telephone is used to attempt to make an outgoing call or a call to another one of the telephones. When detecting a failure of the PBX, the control logic in a fallback detector will actuate the relay in the fallback detector to its fallback positions so as to connect the corresponding telephone directly to the telephone line and allow the telephone to make and receive telephone calls as an analog telephone. In this manner, when a power failure occurs or when the PBX fails in the LAN system, the system is switched to operate in the analog mode and telephone service is maintained even though a power failure or PBX failure has occurred.